This invention relates to a method and apparatus for producing a digital depiction of a signal, in particular for producing an adaptive digital depiction.
Conventional analogue to digital converters (ADCs) are well known. These enable an analogue input signal to be converted into a digital depiction of the input signal.
Various types of ADC are known. Possibly the simplest is the zero crossing discriminator wherein the output changes from a logical naught to a logical one when the output crosses a reference voltage of zero volts. More sophisticated ADCs comprise a number of threshold crossing discriminators each with its own reference voltage, where adjacent reference voltages are spaced apart by a common amount or have some other means for achieving an equivalent effect. For example the outputs of sixteen discriminators expressed as a binary code would indicate that the input voltage lay somewhere between two particular reference voltages. However, for any significant amount of noise at the input, the input signal cannot be considered static during the conversion processes causing the output digital depiction to change rapidly and be almost indeterminate. To overcome this problem the input signal is held constant using a xe2x80x98track and holdxe2x80x99 circuit and the resulting unambiguous output digital depiction during the xe2x80x98hold phase is latched into a register by sample pulses which occur at equally spaced intervals of time.
Analogue to digital conversion in which the input analogue signal is periodically sampled at a predetermined constant rate and covered into a digital depiction are standard. To convert an analogue signal having high frequency components a higher sampling rate must be used resulting in an increased amount of output digital information. Further the high sampling rate results in an increased amount of unnecessary digital information for sections of the analogue input which have a relatively low frequency. For analogue signals having both high and low frequency components a low sampling rate is not appropriate as the high frequency components cannot be then correctly identified. Conventionally the choice of regular sampling rate is subject to the well known Nyquist sampling criteria i.e. that the rate of sampling should be greater than twice the maximum frequency component of the signal.
An alternative to sampling at a constant rate is described in International Patent Application No. PCT/US98/7592. When the input signal changes in amplitude by an amount that exceeds a predetermined threshold level a digital message is output containing information about the change in amplitude, the polarity of the change and the elapsed time for the change. Therefore samples are only taken when the signal itself changes by a predetermined amount and as such the sampling rate is determined by the signal itself. This is an example of an adaptive sampling technique. As used in this specification the term xe2x80x98adaptivexe2x80x99 shall be used to indicate a sampling system wherein the generation of the digital output is based on the evolution of the signal.
Also the system described in PCT/US98/27592 is a system which requires feedback. The crossing of a threshold level causes the levels of the apparatus to be reset. This need to change levels means that the system is inherently slow compared with a flow through system, such as a conventional ADC.
U.S. Pat. No. 4, 680, 797 describes a coder for secure communication of speech which determines certain critical points within a signal and determines the vectors between key points and transmits those vectors. The paper by Balasubramanian K et al. (xe2x80x9cA Novel Method for Optimal Sampled Data Selection, Transmission and Reconstruction: A Proposalxe2x80x9d 1995 IEEE Instrumentation and Measurement Technology Conference. IMTC/95 Waltham Mass. Apr. 23-26, 1995 Proceedings of the Instrumentation and Measurement Technology Conference) similarly discloses a system which identifies key points in the signal and transmits samples only at those points. Both of these systems require analysis of the signal to determine key or critical points however.
Honary B et al: xe2x80x9cAdaptive-Rate Sampling Applied to the Efficient Encoding of Speech Waveformsxe2x80x9d National Conference on Telecommunications, York, 2-5 Apr., 1989, London IEE, vol. CONF 2, 2 Apr. 1989, pages 352-357 teaches a system where the signal is analysed and, based on that analysis the sampling rate is varied accordingly to ensure accurate sampling. The resultant signal is complex however and the bandwidth of the signal needs to be quickly assessed.
It is therefore an object of the present invention to provide a method and apparatus for producing a digital depiction of a signal which is adaptive and which mitigates at least some of the aforementioned disadvantages.
According to the present invention therefore there is provided an apparatus for producing a digital depiction of a signal comprising a sampling means for sampling the signal at a constant rate determined by a clock and providing a first digital depiction containing information regarding the signal amplitude at each sampling time, and a transformation means responsive to the first digital depiction for producing a second digital depiction, characterised in that the transformation means is capable of determining that the signal has changed in amplitude by a certain threshold amount and determining the elapsed time for the signal to change by said amount and producing the second digital depiction based on the elapsed times for the signal to change by predetermined amounts, and in that the sampling rate is set such that the signal varies by less than the predetermined amount between successive samples.
This is an example of adaptive sampling wherein the evolution of the signal, as represented by the first digital depiction, is used to determine what adapted depictions are output.
By using the evolution of the signal itself to produce the second digital depiction the output of the second digital signal can be significantly lower than the first digital depiction. For example when the rate of change of the signal is low that fact can be used to produce a more compact digital depiction with less information than the first digital depiction. The first digital depiction is sampled at a constant rate and so will over sample a low frequency component of the signal but the second digital depiction can avoid this.
Also the second digital depiction could present the information about the signal in a form which is more suited for a particular application. For instance in waveform matching applications it may be desired to match similar waveforms evolving in different time scales. The constantly sampled first digital depiction has a temporal dependence on the sampling process but the second digital depiction, using the signal evolution, can avoid this.
Further, by using a sampling means run at a constant rate to generate a first digital depiction high speed components and conventional digital sampling technology can be used and the adaptive part of the sampling can be carried out in the digital regime.
Preferably the second digital depiction is a digital representation. As used throughout this specification the term xe2x80x98digital depictionxe2x80x99 shall be taken to mean any digital output which contains information about the signal. The term xe2x80x98digital representationxe2x80x99 however shall be taken to mean a representation that digitally describes a signal in such a way that the signal could be reproduced directly by feeding the depiction to a suitable reconstituter, such as a digital to analogue converter, without requiring any preliminary processing. The digital messages produced in PCT/US98/27592 for instance would require prior processing and so do not constitute a digital representation.
In one convenient embodiment the transformation means is capable of determining that the signal has crossed a predetermined threshold level and determining the time interval between crossing a first predetermined threshold level and a second different predetermined threshold level and producing a second digital depiction of the signal. Conveniently the transformation means includes a timer counter means for counting the number of cycles of a clock signal to determine the time interval between predetermined threshold level crossings.
By measuring the time interval between the signal crossing predetermined threshold levels the rate of change of the signal is used to determine when samples are taken. For instance when the signal is changing slowly there is no redundant sampling, simply a longer interval is recorded. This results in the second digital depiction having far fewer digital outputs than the first digital depiction.
The sampling means takes samples at a regular rate determined by the clock. The output of the sampling means gives an estimate of what the amplitude of the signal is at the time at which it is sampled. The transformation means looks at the first digital depiction and from that signal determines when a predetermined threshold level has been crossed and starts timing the time interval until the transformation means determines that a different predetermined threshold level has been crossed.
The time at which the signal crossed the predetermined threshold level will not be known exactly. However the time intervals are measured digitally by counting the number of cycles of a clock that occur between the level crossings. This process inherently involves a maximum error in the measurement of the interval equal to the time between clock cycles. The error in the measurement of the interval will dominate any error in determining when a predetermined threshold was crossed. Because of this error it is not necessary to know exactly when a predetermined threshold level is crossed.
Preferably the clock signal driving the sampling means and transformation means are derived from the same clock. Using signals derived from the same clock to drive both the sampling means and the transformation means ensures no error is introduced because the clocks are unsynchronised which reduces the error of the system compared with using entirely independent clock signals. If independent clock signals are used for the sampling means and the transformation means the dock signal of the transformation means is preferably faster that that of the sampling means. Where the clock signals are derived from the same clock the clock signals preferably run at the same rate. However this will give a minimum interval indication of either 1 or 0 depending on how the system is set up. If a different number were required for the minimum interval the clock of the transformation means could be faster by a factor of K, where K is the desired number for the minimum interval.
Preferably the sampling means comprises an analogue to digital converter. Fast ADCs are readily available. Conveniently the ADC used could be a sampled flash ADC.
Preferably the sampling means includes an anti aliasing filter which serves to limit the fastest rate of change of the input signal.
Conveniently the transformation means comprises an arrangement of logic gates. Probably the transformation means would comprise a suitably programmed Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC).
The second digital depiction may consist of information about the elapsed time between predetermined threshold level crossings. Advantageously the transformation means may also comprise means for determining in which direction the signal has crossed a predetermined threshold level and wherein the second digital depiction includes an UP/DOWN digital output signal to indicate in which direction, UP or DOWN, the signal has crossed the predetermined threshold level.
In a second aspect of the present invention there is provided a method of producing a digital depiction of a signal comprising the steps of sampling a signal at a constant rate determined by a first dock signal to provide a first digital depiction with information about the amplitude of the signal at each sampling time characterised in that it further comprises the steps of;
monitoring said first digital depiction to determine when the signal has changed in amplitude by a certain threshold amount, and
determining the elapsed time for the signal to change by said amount and producing a second digital depiction based on the elapsed times for the signal to change by predetermined amounts, and in that
the step of sampling the signal involves sampling the signal at a rate that is set such that the signal varies by less than the predetermined amount between successive samples.
Preferably the second digital depiction is a digital representation.
In one particular embodiment the method includes the steps of determining when a predetermined threshold level has been crossed, determining the elapsed period of time between the signal crossing a first predetermined threshold level and crossing a second different predetermined threshold level using a second clock signal, and providing a second digital depiction of the signal.
Preferably the rate of sampling the signal is sufficient such that the signal will not cross more than one predetermined threshold level between successive samples. This is important when a digital representation is to be produced.
Conveniently the method includes the step of determining the direction of change of the signal as it crosses a predetermined threshold level and providing a digital output of the direction of change, UP or DOWN in the second digital depiction.
Preferably the second clock signal is derived from the same clock as the first clock signal. Conveniently the first and second clock signals run at the same rate. As a less preferable alternative the first and second clocks signals may be derived from different clocks in which case the second dock signal is advantageously run faster than the first clock signal.
The present invention can be applied to existing conventional ADCs to allow for adaptive sampling. Therefore in a third aspect of the present invention there is provided a method for adapting a constant sampling rate ADC to carry out adaptive sampling comprising the steps of taking an analogue to digital converter that samples a signal at a constant rate in response to a clock signal, and introducing the output of the analogue to digital converter to a transformation means characterised in that the transformation means is responsive to the output of the analogue to digital converter and is capable of determining when the signal has changed in amplitude by a certain threshold amount and determining the elapsed time for the signal to change by said amount and producing a second digital depiction based on the elapsed times for the signal to change by predetermined amounts, and in that the sampling rate is set such that the signal varies by less than the predetermined amount between successive samples.
Preferably the second digital depiction is a digital representation.
In one example the transformation means is capable of determining when a predetermined threshold level is crossed by the signal, determining the elapsed time between this crossing and the signal crossing another predetermined threshold level, and producing a digital depiction of the signal.
Conveniently the method includes the step of supplying a clock signal to the transformation means from the same clock that controls the sampling rate of the analogue to digital conversion.
Preferably the transformation means comprises an arrangement of logic gates. Conveniently the transformation means includes a timer counter.
Preferably the sampling rate of the analogue to digital conversion means is adapted such that the signal crosses no more than one predetermined threshold level between successive samples.